1. Field of the Invention
The present invention is directed to a process for preparing an oil-in-water emulsion. More particularly, the present invention is directed to a process for preparing a stable oil-in-water emulsion of fine particle size which is useful in making emulsions for use in woven and non-woven textiles, cosmetic, personal care and pharmaceutical applications.
2. Prior Art
U.S. Pat. No. 4,620,878 describes a method for making a polyorganosiloxane fine emulsion or microemulsion. In this method a translucent oil concentrate is prepared. The concentrate is rapidly dispersed in water by the incremental addition of water to the concentrate to form an oil-in-water fine or microemulsion using high-shear mixing. This patent explicitly teaches: "When a single nonionic surfactant is employed as the insoluble surfactant, the cloud point of the surfactant should be higher than the temperature at which the emulsion is prepared." (Col. 9, lines 37-40).
S. Friberg, and K. Shinoda, Emulsions and Stabilization (John Wiley & Sons, 1986), report two distinct methods of emulsion preparation: Emulsification by the Phase Inversion Temperature (PIT) Method and Emulsification by the Inversion Process (IP). PIT emulsification involves preparing an emulsion near (just below) the phase inversion temperature of the system. The phase inversion temperature is a characteristic property of an emulsion (rather than of the surfactant alone) at which the hydrophile-lipophile property of the nonionic surfactant is in balance (i.e., the temperature of the emulsion at which the surfactant has an equal affinity for both the oil and water phases of the emulsion). According to the PIT method, desired emulsions are obtained if an emulsion system is initially emulsified just below the PIT and then cooled rapidly, since interfacial tension is reduced at the PIT and rapid cooling adds small droplets from the initial phase separation of the surfactant.
In the IP method of Friberg and Shinoda, emulsification is performed at a temperature higher than the PIT in order to first form a water-in-oil emulsion which then inverts to an oil-in-water emulsion upon cooling below the PIT. That is, Friberg and Shinoda emphasize either conducting the emulsification close to or slightly below the cloud point (PIT) or forming a water-in-oil emulsion at elevated temperature which is then inverted upon cooling (IP).
Surprisingly, it has been found that, by the process of the present invention, oil-in-water emulsions are easily obtained without the temperature constraints of the prior art methods. Further, the degree of mechanical agitation or work required to prepare the emulsion is significantly reduced by the process of this invention as compared to the above-mentioned processes in the art. Additionally, the process of the present invention permits the manufacture of emulsions which have a low bio-burden, since the emulsion is optionally pasteurized during preparation, thereby reducing the bio-burden that must be overcome by the addition of antimicrobial preservatives. Further, the process of the present invention produces stable (i.e., shelf life or storage life of at least two weeks) oil-in-water emulsions having a fine particle size.